Liquid filter with separate and calibrated vapor release

ABSTRACT

A liquid filter ( 10 ) housing an annular filter element ( 16 ) has an annular space or inlet chamber ( 24 ) between the filter element and the housing and viewable through the housing such that an operator can see the level of liquid in the inlet chamber as an indication of when to replace the filter element, the higher the level of liquid in the inlet chamber the greater the pressure drop across the filter element. A change interval plugging indicator ( 102 ), preferably part of the filter element, is provided by a gas trap and pressure responsive release mechanism ( 104 ) trapping gas in the upper section ( 23 ) of the inlet chamber until a designated release pressure is reached, corresponding to a desired terminal pressure, to prevent premature plugging indication otherwise indicated by rising liquid level in the inlet chamber. Accurate and reliable calibration is enabled by separating the filtration function and the plugging indication function.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is a continuation-in-part of U.S. patentapplication Ser. No. 09/934,576, filed Aug. 22, 2001.

BACKGROUND AND SUMMARY

[0002] The invention relates to liquid filters, and more particularly toa service interval change indicator more accurately reflecting filterlife.

Parent Application

[0003] Liquid filters, including fuel filters, typically have avertically axially extending housing having an annular filter elementextending axially between top and bottom ends and having an inner hollowinterior and an outer annular space between the filter element and thehousing. The housing has an inlet to the annular space, and an outletfrom the hollow interior. Liquid is filtered by flowing from the annularspace through the filter element into the hollow interior. The annularspace is viewable through the housing, e.g. through a transparenthousing side wall, such that an operator or service technician can seethe level of liquid in the annular space as an indication of when toreplace the filter element. The higher the level of liquid in theannular space the greater the pressure drop across the filter elementand hence the greater the plugging of the filter element.

[0004] In many applications, the liquid or fuel level, including therise thereof in the noted annular space, does not accurately reflectfilter life. For example, in one application, fuel level in a clearhousing reaches the top with 2″ Mercury, Hg, (68,000 dyne/cm²),restriction, while the filter element is capable of 8″ Mercury (271,000dyne/cm²), restriction. Hence, using fuel level in the noted annularspace of the clear housing as an indicator to change the filter elementresults in a premature such change. This is objectionable because of theless than full life usage of the filter element, the more frequentfilter element changes, and the corresponding higher overall costthereof.

[0005] The invention of the noted parent application addresses andsolves the above noted objections. In one aspect, the parent inventiondelays the rise in liquid level in the noted annular space to correctthe otherwise premature indication of a need to change the filterelement. In another aspect, liquid level in the noted annular space isallowed to rise to controlled levels providing advance and more accurateindication of a forthcoming need to change the filter element.

Present Invention

[0006] The present invention arose during continuing development effortsdirected toward liquid filters, including fuel filters, including fordiesel fuel, which must provide high removal efficiency and low pressuredrop.

[0007] As is known, fuel filters are often used on the suction side (lowpressure or vacuum side) of the fuel pump. In order to obtain reasonableservice intervals without fuel starving the engine, the initial pressuredrop should be under 1 inch of Mercury, Hg, (34,000 dyne/cm²), with aterminal pressure drop across the filter, ΔP, in the range of 5 to 10inches of Mercury (170,000 to 340,000 dyne/cm²). Historically, pluggedfuel filters are one of the leading causes for on-highway service callsfor over-the-road trucks. Typically, the filters are plugged with softcontaminants, e.g., asphaltenes, biological growth, resins, or othersludge-type material, often introduced as a result of fueltransportation and handling. At the same time, truck operators are undereconomic pressure to reduce costs, including maintenance costs due toscheduled and unscheduled maintenance and replacement filters. It isthus desirable for operators to be able to fully utilize thecontaminant-holding capacity of filters (i.e. not change out the filterstoo soon), without actually plugging the filter and jeopardizing engineoperation. To do this, they must be able to accurately detect filterplugging. In the past, a variety of gages and sensors have been utilizedto monitor pressure drop across the filter, however these are expensive,not always reliable, and are often ignored by operators. Another commontechnique is to use air/fuel vapor in the fuel filter housing as avisual indicator of element condition. In the air visualization method,the disappearance of the air is erroneously assumed to be an indicatorof filter plugging. The underlying premise for this method is theobservation that, in practice, air carried along with the fuel tends tocollect in the top of the filter housing. This air pocket is commonlyobserved with new filters and not with plugged filters. The airvisualization method is relatively popular, in that it is inexpensive,easy and, if used regularly, minimizes problems resulting from waitingtoo long to change the filter. On the other hand, the method isinaccurate and leads to premature, costly filter changes.

[0008] The present invention addresses and provides further solutions tothe above noted objections. In one aspect, the invention provides animproved air visualization method and mechanism avoiding the above notedsource of inaccuracy. In one form, this is accomplished by separatingthe filtration and indication functions. Various embodiments of thelatter are provided.

BRIEF DESCRIPTION OF THE DRAWING Parent Application

[0009] FIGS. 1-8 are taken from parent U.S. patent application Ser. No.09/934,576.

[0010]FIG. 1 is a side elevation view of a liquid filter known in theprior art.

[0011]FIG. 2 is a sectional view taken along line 2-2 of FIG. 1.

[0012]FIG. 3 is a perspective view of a filter element in accordancewith the parent invention.

[0013]FIG. 4 is a view like FIG. 1 but incorporating the filter elementof the parent invention.

[0014]FIG. 5 is a view like FIG. 3 and shows a further embodiment.

[0015]FIG. 6 is a view like FIG. 2 but incorporating the filter elementof FIG. 5.

[0016]FIG. 7 is like FIG. 6 and shows a further stage of operation.

[0017]FIG. 8 is like FIG. 5 and shows a further embodiment.

Present Invention

[0018]FIG. 9 is a graph showing field test data for fuel filters knownin the prior art.

[0019]FIG. 10 is a view like FIG. 3 and shows a filter element inaccordance with the present invention.

[0020]FIG. 11 is a view like FIG. 6 but incorporating the filter elementof FIG. 10.

[0021]FIG. 12 is a view like portion of FIG. 11 and shows a furtherembodiment.

[0022]FIG. 13 is a view like FIG. 10 and shows a further embodiment.

[0023]FIG. 14 is a schematic illustration of a filter element inaccordance with the invention, illustrating operation.

[0024]FIG. 15 is a graph illustrating performance.

DETAILED DESCRIPTION OF THE INVENTION Parent Application

[0025] The following description regarding FIGS. 1-8 is taken fromparent U.S. patent application Ser. No. 09/934,576.

Prior Art

[0026]FIG. 1 shows a liquid filter 10, for example a diesel fuel filter,known in the prior art. The filter includes a housing 12 extending alonga generally vertical axis 14 and having an annular filter element 16extending axially between top and bottom ends 18 and 20 at respectiveupper and lower end caps 19 and 21, FIG. 2, and having an inner hollowinterior 22, FIG. 2, and an outer annular space 24 between filterelement 16 and side wall 26 of housing 12. The housing has a lower inlet28, FIG. 2, to annular space 24, and a lower outlet 30 from hollowinterior 22 through outlet tube 32. In the case of a diesel fuel filter,the housing may include a lower collection bowl or reservoir 34 forcollecting coalesced separated water or contaminants for drainage atdrain outlet 35 as controlled by valve 36, and may have an electricalconnection 38 for an internal heater, and so on, as is known.

[0027] Housing 12 includes the noted cylindrical sidewall 26 closed atits top end by upper end cap 40 in threaded relation, and closed at itsbottom end at housing base 42 in threaded relation. Sidewall 26 is clearor transparent, and hence annular space 24 is viewable through thehousing such that an operator or service technician can see the level ofliquid such as 44, FIG. 1, in annular space 24. When the liquid risesfrom level 44, as shown in dashed line in FIG. 2, to level 46, theoperator or service technician can see such level and the change thereofas an indication of when to replace filter element 16. The higher thelevel of liquid in annular space 24 the greater the pressure drop acrossfilter element 16 and hence the greater the plugging of filter element16. Unfortunately, it has been found in numerous applications that suchliquid level rise from 42 to 46 does not correspond to expired filterlife. Hence, the filter element is being changed prematurely, and has alonger life than otherwise indicated by the noted rising liquid level inannular space 24.

Parent Invention

[0028] FIGS. 3-8 illustrate the parent invention and use like referencenumerals from above where appropriate to facilitate understanding. Theparent invention is illustrated in the context of the above noted knowndiesel fuel filter, though the parent invention is not limited thereto.

[0029] The parent invention delays the rise of fluid level in annularspace 24 for applications where filter element 16 is otherwise changedprematurely and has a longer life than otherwise indicated by the notedrising liquid level in annular space 24. The liquid gives off vapor inthe housing, including in annular space 24. This aspect is utilized inthe parent invention. The noted delay is provided by a delay member inthe form of a vapor and liquid impermeable sleeve 50 around filterelement 16 and having a top end 52 at the top end of the filter elementand having a bottom end 54 spaced from the bottom end of the filterelement by an axial gap 56. The sleeve has an outer face 58, FIGS. 4, 6,facing annular space 24, and an inner face 60 facing filter element 16.Liquid and vapor flow from annular space 24 radially inwardly throughaxial gap 56 and radially inwardly through filter element 16 thereat,and also flow axially along inner face 60 of sleeve 50 and radiallyinwardly through filter element 16 thereat. When the liquid level inannular space 24 rises above bottom end 54 of sleeve 50, as shown atlevel 62, FIGS. 4, 6, vapor above level 62 can no longer flow throughaxial gap 56 and is trapped in annular space 24 above bottom end 54 ofsleeve 50 due to the vapor impermeability of sleeve 50. Further rise ofliquid level in annular space 24 must compress trapped vapor therein,thus slowing and delaying the rise of liquid level in annular space 24.

[0030] In a further embodiment, FIG. 5, sleeve 50 has one or moreapertures therein such as 70, 72, 74, etc. at respective given locationstherealong, each having a respective liquid-soluble button 76, 78, 80,respectively, for example a fuel-soluble button made of polyisobutylene,for example available from Lubrizol under Part Number OS158536. Thebutton initially closes the respective aperture, and then is dissolvedafter a given time by contact with the liquid flowing along inner face60 of sleeve 50, such that vapor in annular space 24 may pass throughthe aperture vacated by the button, FIG. 6, whereafter the liquid levelrises in annular space 24 from level 62, FIG. 6, to level 82, FIG. 7.The noted given time is preferably selected to be the filter elementchange interval, e.g. by matching dissolution rate of the materialand/or thickness to the desired interval. Upon dissolution of the buttonand passing of vapor from annular space 24 through the respectiveaperture in sleeve 50, the rising liquid level in annular space 24provides an indication to the operator to change filter element 16.

[0031] In a further embodiment, FIG. 8, sleeve 50 has a furtherplurality of apertures such as 90, 92 each filled with a respectiveliquid-soluble button 94, 96 and axially spaced from bottom end 54 ofsleeve 50 by differing axial spacings. Buttons 96, 94, 78 have differingdissolution rates, e.g. by differing thicknesses and/or differingmaterial selection. A first of the buttons such as 96 closest to bottomend 54 of sleeve 50 has the fastest dissolution rate and dissolves firstsuch that liquid level in annular space 24 rises to a respective firstaperture 92 vacated by first button 96. This provides a first advanceindication of a forthcoming need for a filter element change. A secondof the buttons such as 94 is spaced axially farther from bottom end 54of sleeve 50 than first button 96 and has a slower dissolution rate anddissolves second such that the liquid level in annular space 24 furtherrises to a respective second aperture 90 vacated by second button 94.This provides a second sequential indication of an oncoming need for afilter element change.

Present Invention

[0032] In the above noted prior art, the assumption underlying the notedair visualization method is that is that the increased pressure dropacross the filter causes the air to pass through the filter uponplugging. The pressure drop needed for the air to flow through thefilter media is given by:

ΔP=ΔP_(1,A)  Equation (1)

[0033] where ΔP is the pressure drop across the filter element (atplugging in this case) and ΔP_(1,A) is the capillary pressure requiredfor air to flow through the filter media, as given by the LaPlaceEquation, also known as the bubble point of the media. ΔP_(1,A) is givenby: $\begin{matrix}{{\Delta \quad P_{1,A}} = \frac{4\gamma \quad \cos \quad \theta}{D}} & {{Equation}\quad (2)}\end{matrix}$

[0034] where γ is the surface tension of the fuel, θ is the three-phasecontact angle of the filter media-fuel-air, and D is the maximum porediameter of the filter media. At the bubble point, when air starts todisplace the oil in the filter media, θ≈0°. For typical fuel, γ isapproximately 25 dyne/cm.

[0035] In application, D corresponds to the maximum pore diameter, sinceair (and fuel) will flow through the least restrictive opening. Allfilter media has a distribution of pore sizes. As a filter plugs, thereis a net decrease in the average pore size, due to contaminant capture,however the maximum pore size changes little, as most contaminantbuild-up occurs in the smaller pores that remove most of thecontaminant. Typical fuel filter media bubble points are on the order of25,000 dyne/cm² (0.7 inches of Mercury), corresponding to D=40 μm, whilethe terminal pressure drop is 170,000 to 340,000 dyne/cm² (5 to 10inches of Mercury), corresponding to 6 μm<D. It is unlikely that theseleast restrictive pores would remove enough contaminant to reduce insize by a factor of 6 or more. There is no correlation between ΔP_(1,A)(the pressure drop at which the air pocket starts to disappear) and ΔP(the terminal pressure drop), as shown in FIG. 9. FIG. 9 shows theactual pressure drop across the filter (abscissa) plotted against thefuel height (traditional prior art air visualization method). The datawere obtained from a field test on over-the-road trucks using twodifferent types of diesel fuel filters. The pressure drop across thefilter and fuel height were monitored during the test. According toindustry standards, a filter is plugged at 271,000 dyne/cm² (8 inches ofMercury) pressure drop. At 203,000 dyne/cm² (6 inches of Mercury), it isrecommended that users replace the filter as soon as practical. Usingthe traditional air visualization method, users would consider thefilter plugged when the fuel height exceeds 13.3 cm (5.25 inches). Atthis height, the air pocket disappears. In the data of FIG. 9, thetraditional air visualization method indicated plugging for 36 of 113data points. However, the filter was actually plugged only 3 times andin the “replace filter” region 2 additional times. The traditional airvisualization method prematurely indicated filter plugging 27% of thetime. This results in premature filter replacement and added expense forengine operators, and demonstrates the inaccuracy of the traditional airvisualization method.

[0036] The problem with the noted traditional air or gas visualizationmethod is that the filtration function and the plugging indicationfunction both occur in the filter media and are affected by the samefactors. The air visualization method requires that a change in the verylargest pores corresponds to what is occurring in the rest of the media.In accordance with the present invention, a way to avoid this source ofinaccuracy is to separate the two functions. This is accomplished in thepresent invention, and provides more accurate and reliable detection offilter element plugging than the traditional air visualization method.

[0037]FIGS. 10 and 11 use like reference numerals from above whereappropriate to facilitate understanding. Liquid filter 10, for examplethe noted diesel fuel filter, houses annular filter element 16 having anupstream outer face 15 communicating with inlet 28, and a downstreaminner face 17 communicating with outlet 30. Filter element 16 filtersliquid passing radially inwardly therethrough from outer face 15 toinner face 17. Housing 10 has the noted sidewall 26 which defines withouter face 15 the noted annular space or inlet chamber 24 therebetweenhaving an upper section 23 and a lower section 25. As liquid such asfuel enters inlet chamber 24 from inlet 28, gas or air in the liquidrises to upper section 23 of inlet chamber 24. Inlet chamber 24 isviewable through the housing, as noted above as provided by a clear ortransparent sidewall 26, such that an operator can see the level ofliquid in inlet chamber 24. A low level of liquid indicates a lowpressure drop initial non-plugged condition of filter element 16. Thehigher the level of liquid in inlet chamber 24 the greater the pressuredrop across filter element 16 and the greater the plugging of the filterelement.

[0038] A change interval plugging indicator 102 is provided in thehousing by a gas trap and pressure responsive release mechanism 104trapping gas in upper section 23 of inlet chamber 24 until a designatedrelease pressure is reached, corresponding to a desired terminalpressure, to prevent premature plugging indication otherwise indicatedby rising liquid level in the inlet chamber. The change intervalplugging indicator is preferably part of the filter element and includesouter wrap 50 around outer face 15 of filter element 16 in upper section23 of inlet chamber 24 and blocking gas flow therethrough at least atpressures below the noted designated release pressure. Outer wrap 50 hasthe noted lower end 54 which defines a lower end of upper section 23 ofinlet chamber 24 and an upper end of lower section 25 of the inletchamber. Gas is trapped in upper section 23 of the inlet chamber whenliquid in the inlet chamber rises above lower end 54 of outer wrap 50.

[0039] Filter element 16 is the noted axially extending annulus havingan outer surface providing the noted outer face 15, and having an innersurface providing the noted inner face 17 and defining inner hollowinterior 22. The filter element has the noted lower axial end 21 havingan opening 31 at hollow interior 22 and communicating with outlet 30.The filter element has the noted upper axial end 19. A check valve 106is provided at the upper axial end of the filter element and has a firstside 108 communicating with hollow interior 22, and a second side 110communicating with upper section 23 of inlet chamber 24 for examplethrough one or more radially extending gas passages 112 formed betweenspokes such as 114 in upper end cap 40. Spokes 114 apply axial pressureagainst filter element 16 holding the latter in place. Check valve 106has a closed condition below the designated release pressure, and anopen condition above the noted designated release pressure. In theembodiment of FIG. 11, check valve 106 is provided by porous media atupper axial end 19 of the filter element covering hollow interior 22 andblocking gas flow therethrough below the designated release pressure,and passing gas flow therethrough above the designated release pressure.In an alternate embodiment, FIG. 12, the check valve is provided by abiased valve member 116, such as a ball, flap, flanged stem, or thelike, biased upwardly by bias member 118, such as a spring, to anormally closed condition below the noted designated pressure, e.g. asseated against and closing opening 120 at valve seat 122. The valve hasan open condition overcoming the noted bias above the noted designatedrelease pressure, e.g. to move ball 116 downwardly against the bias ofspring 118 to permit gas to flow downwardly through opening 120 andthrough vents such as 124 into hollow interior 22.

[0040] In the embodiments of FIGS. 10-12, outer wrap 50 is preferablyrelatively non-porous, i.e. completely impervious to gas and liquidflow, or having a permeability substantially lower than filter element16. In another embodiment, outer wrap 50 is a porous member wetted bythe liquid in inlet chamber 24 such that capillary pressure in suchporous member blocks gas flow therethrough below the noted designatedrelease pressure, and such that pressure above such designated releasepressure overcomes the capillary pressure, and gas passes through outerwrap 50, to be described. In this latter embodiment, upper end cap 19 offilter element 16 is engaged in sealing relation by upper end cap 40 ofthe housing, FIG. 6, such that no gas passes therebetween from inletchamber 24 to hollow interior 22. In an alternative, FIG. 13, upper endcap 19 a of filter element 16 is a solid member spanning hollow interior22 and having no opening thereinto.

[0041] In each of the described embodiments, the gas trap and pressureresponsive release mechanism delays rise in liquid level in inletchamber 24 for applications where filter element 16 is changedprematurely and has a longer life than otherwise indicated by risingliquid level in inlet chamber 24. Filter media 16 a of filter element 16performs a filtration function by passing liquid therethrough. Duringuse, liquid level in inlet chamber 24 rises, and gas in the inletchamber disappears as permitted by gas flow through filter media 16 a.In the prior art, the rising liquid level and the disappearing gas isused for indicating a change interval for filter element 16. In thismanner in the prior art, filter media 16 a provides both a filtrationfunction and a plugging indication function. The present system providesan improved gas visualization interval change plugging indication methodby separating the filtration function and the plugging indicationfunction by trapping gas in inlet chamber 24, and then releasing the gasin response to a designated release pressure corresponding to a desiredterminal pressure. During use, the pressure drop across filter media 16a increases as the latter becomes more restrictive to liquid flowtherethrough as more contaminant is captured. In the present invention,the designated release pressure is independent of and does not vary withthe increasing restriction of filter media 16 a to liquid flowtherethrough. The filtration function is performed with a first memberprovided by filter media 16 a. The plugging indication function isperformed with a second member different than the noted first member. Inpreferred form, the noted second member is provided by outer wrap 50 andin some embodiments also in combination with check valve 106. The flowproperties of the first member 16 a vary during filtration. The flowproperties of the second member 50 do not substantially vary duringfiltration. In one preferred embodiment, the second member is providedby outer wrap 50 around outer face 15 of filter element 16, the outerwrap having a lower permeability than filter media 16 a, and wherein thedesignated release pressure is calibrated to correspond to the noteddesired terminal pressure according to bubble point of the outer wrap,to be described. The noted gas trap and pressure responsive releasemechanism 104 traps gas in inlet chamber 24 until the noted designatedrelease pressure corresponding to the noted desired terminal pressure isreached, and then releases the gas to escape to outlet 30 through gaspassage 112 and hollow interior 22, and/or through outer wrap 50 andfilter media 16 a and hollow interior 22, to outlet 30, which escapinggas is replaced by increasing liquid levels in inlet chamber 24,indicating that the designated release pressure corresponding to thedesired terminal pressure has been reached, which in turn indicates thatreplacement of filter element 16 is due.

[0042]FIG. 14 is a schematic illustration and uses like referencenumerals from above where appropriate to facilitate understanding. Fuelcan flow around outer wrap 50 and through filter media 16 a. An airpocket forms and is maintained outside the outer wrap, where it can bedetected visually if a transparent housing or sight glass is used. Fordiscussion and modeling purposes, the element is divided into twosections: Section 1 (the portion above lower end 54 of outer wrap 50)and Section 2 (portion below end 54). The advantage of this invention,compared to the prior art air visualization method, is that the use ofan outer wrap separates the filtration function (of the filter media)from the plugging detection function. It uses capillary pressure toprevent air flow until the terminal pressure drop is achieved. Unlikethe prior art air visualization method, the properties of the outer wrapdo not change significantly as filtration occurs, and an accuratedetermination for filter plugging can be made. In contrast, the priorart air visualization method requires that a change in the very largestpores corresponds to what is occurring in the rest of the media.

[0043] Section 1 refers to the portion of the filter element covered bythe outer wrap. First, we assume that the axial variation in flow rate,pressure and pressure drop across the filter media in this section isnegligible. Based on this assumption, the pressure drop across thefilter element (ΔP) is given by the following equation:

ΔP=ΔP _(1,B) +ΔP _(1,A)  Equation (3)

[0044] where ΔP_(1,B) is the pressure drop caused by the outer wrap, andΔP_(1,A) is the pressure drop across the filter media. In the initialstate of the filter, ΔP_(1,B) is due to the restriction across the outerwrap and to the restriction caused by flow through the channel formedbetween the filter media and the outer wrap. These two contributions arelumped together. For purposes of what we are trying to achieve, it isnot critical to distinguish between them, as can be seen later. It isnoteworthy that in this initial state, there is no airflow through theouter wrap. ΔP_(1,A) is due to the pressure drop across the new media.In the final state of the filter, at the precise moment when air flowsthrough the outer wrap, ΔP_(1,B) is equal to the bubble point of theouter wrap in the specific fuel, and ΔP_(1,A) is due to the pressuredrop across the plugged media.

[0045] Section 2 refers to the portion of the filter element uncoveredby the outer wrap. Again, we assume that the axial variation in flowrate, pressure and pressure drop across the filter media in this sectionis negligible (a more accurate assumption in this section compared tosection 1). The pressure drop across the filter element is given by thefollowing equation:

ΔP=ΔP_(2,A)  Equation (4)

[0046] where ΔP_(2,A) is the pressure drop across the filter media. Inthe initial state of the filter, ΔP_(2,A) is due to the pressure dropacross the new media. In the final state of the filter, ΔP_(2,A) isequal to the pressure drop across the plugged media. It is noteworthythat ΔP_(1,A) ≠ΔP_(2,A) in neither the initial nor final state, as theflow through section 1 also includes restriction contributions from theouter wrap and channel.

[0047] Now considering the filter element as a whole assuming constanttotal flow rate through the filter element, the total flow rate (Q) isgiven by:

Q=Q ₁ +Q ₂  Equation (5)

[0048] where Q₁ is the flow rate through section 1, and Q₂ is the flowrate through section 2. As the filter plugs, Q₂/Q decreases, Q₁/Qincreases, and both ΔP_(1,A) and ΔP_(2,A) increase. One implication ofthis is that different amounts of contaminant (per unit area) willaccumulate on each section of filter media. At the moment when airstarts to flow through the outer wrap, ΔP_(1,B) due to fuel flow throughthe outer wrap and channel equals the bubble point of the outer wrap inthe specific fuel.

[0049] The challenge is to relate ΔP_(1,B) to the terminal ΔP in state2, i.e., the terminal pressure drop of the plugged filter. The followingfocuses on state 2:

ΔP=ΔP _(1,B) +ΔP _(1,A)  Equation (6)

[0050] $\begin{matrix}{{\Delta \quad P_{1,A}} = \frac{Q_{1}R_{1}}{A_{1}}} & {{Equation}\quad (7)}\end{matrix}$

[0051] where R₁ is a resistance coefficient for the plugged filtermedia, and A₁ is the cross-sectional (face) area of section 1 filtermedia. R₁ is a function of the media properties, contaminant and amountof contaminant deposited. In section 2, $\begin{matrix}{{\Delta \quad P} = {{\Delta \quad P_{2,A}} = \frac{Q_{2}R_{2}}{A_{2}}}} & {{Equation}\quad (8)}\end{matrix}$

[0052] where R₂ is a resistance coefficient for the plugged filtermedia, and A₂ is the cross-sectional (face) area of section 2 filtermedia. R₂ is a function of the same factors as R₁. As an approximation,assume A₁=A₂. In the plugged state, virtually all of the filter media inboth sections will be used, so if we start out with A₁=A₂, this is areasonable approximation. As mentioned previously, different amounts ofcontaminant per unit area will accumulate on the filter media insections 1 and 2. Since the flow through section 1 increases (and hencethe amount of contaminant deposited on the media) as section 2 plugs,these differences are not expected to be great and we can assume thatR₁≈R₂. Hence, $\begin{matrix}{\frac{R_{1}}{A_{1}} = {\frac{R_{2}}{A_{2}} = \frac{\Delta \quad P}{Q_{2}}}} & {{Equation}\quad (9)} \\{{\Delta \quad P} = {{\Delta \quad P_{1,B}} + \frac{Q_{1}\Delta \quad P}{Q_{2}}}} & {E\quad q\quad u\quad a\quad t\quad i\quad o\quad n\quad (10)}\end{matrix}$

[0053] This equation shows that the bubble point of the outer wrap is afunction of the terminal pressure drop and the ratio of flow ratesthrough the two sections. Conversely, the flow rate ratio is a functionof ΔP_(1,B) and ΔP (as well as test conditions e.g., flow rate, fluid,contaminant), as shown by the rearranged equation: $\begin{matrix}{\frac{Q_{1}}{Q_{2}} = {\frac{\left( {{\Delta \quad P} - {\Delta \quad P_{1,B}}} \right)}{\Delta \quad P} = {1 - \frac{\Delta \quad P_{1,B}}{\Delta \quad P}}}} & {{Equation}\quad (11)}\end{matrix}$

[0054] The model, thus far, yields expected behavior. As shown, the flowrate ratio (Q₁/Q₂) is always less than 1 and increases with increasingterminal pressure drop, as expected. At very low ΔP, <ΔP_(1,B), thepressure drop is not high enough to force air flow through the outerwrap and there is, by definition, no terminal pressure drop achieved.

[0055] From a design standpoint, we need to know how the flow rate ratiorelates to terminal pressure drop. By running experiments, in which theouter wrap is varied (hence, ΔP_(1,B)), but not the filter media, filterdesign, or contaminant, the relationship between ΔP and ΔP_(1,B) can bedetermined and used to select an outer wrap corresponding to thepredetermined terminal pressure drop, as shown in FIG. 15. The resultsare noteworthy in that they show that the terminal pressure dropincreases with increasing outer wrap bubble point, as predicted by themodel. Further, they show that outer wraps can be chosen based on bubblepoint and used as reliable indicators of element plugging atpredetermined pressure drops, unlike the prior art air visualizationmethod. For simplicity, linear regression is used to describe therelationship between ΔP and ΔP_(1,B) in the figure. It is interesting tonote that the X-intercept of the regression is a reasonableapproximation of the initial, clean pressure drop across the filtermedia under the test conditions, as expected.

[0056] It is recognized that various equivalents, alternatives andmodifications are possible within the scope of the appended claims. Forexample, as in the parent application, annular includes otherclosed-loop configurations, such as ovals, racetracks, etc.

What is claimed is:
 1. A liquid filter comprising a housing having aninlet and an outlet, a filter element in said housing and having anupstream outer face communicating with said inlet, and a downstreaminner face communicating with said outlet, said filter element filteringliquid passing therethrough from said outer face to said inner face,said housing having a sidewall which defines with said outer face aninlet chamber therebetween having an upper section and a lower section,such that as liquid enters said inlet chamber from said inlet, gas insaid liquid rises to said upper section of said inlet chamber, a lowlevel of liquid indicating a low pressure drop non-plugged condition ofsaid filter element, the higher the level of said liquid in said inletchamber the greater the pressure drop across said filter element and thegreater the plugging of said filter element, a change interval pluggingindicator in said housing comprising a gas trap and pressure responsiverelease mechanism trapping said gas in said upper section of said inletchamber until a designated release pressure is reached, corresponding toa desired terminal pressure, to prevent premature plugging indicationotherwise indicated by rising liquid level in said inlet chamber.
 2. Theliquid filter according to claim 1 wherein said change interval pluggingindicator comprises an outer wrap around said outer face of said filterelement in said upper section of said inlet chamber and blocking gasflow therethrough at least at pressures below said designated releasepressure.
 3. The liquid filter according to claim 2 wherein said outerwrap has a lower end which defines a lower end of said upper section ofsaid inlet chamber and an upper end of said lower section of said inletchamber, and wherein said gas is trapped in said upper section of saidinlet chamber when liquid in said inlet chamber rises above said lowerend of said outer wrap.
 4. The liquid filter according to claim 3wherein said filter element is an axially extending annulus having anouter surface providing said outer face, and having an inner surfaceproviding said inner face and defining a hollow interior, said filterelement having a lower axial end having an opening at said hollowinterior and communicating with said outlet, said filter element havingan upper axial end, and comprising a check valve at said upper axial endof said filter element, said check valve having a first sidecommunicating with said hollow interior and having a second sidecommunicating with said upper section of said inlet chamber, said checkvalve having a closed condition below said designated release pressure,and an open condition above said designated release pressure.
 5. Theliquid filter according to claim 4 wherein said check valve comprisesporous media at said upper axial end covering said hollow interior andblocking gas flow therethrough below said designated release pressure,and passing gas flow therethrough above said designated releasepressure.
 6. The liquid filter according to claim 4 wherein said checkvalve comprises a biased valve member having a normally closed conditionbelow said designated release pressure, and having an open conditionovercoming said bias above said designated release pressure.
 7. Theliquid filter according to claim 4 wherein said outer wrap is nonporous.8. The liquid filter according to claim 7 wherein said outer wrap has apermeability substantially lower than said filter element.
 9. The liquidfilter according to claim 3 wherein said outer wrap is a porous memberwetted by said liquid such that capillary pressure in said porous memberblocks gas flow therethrough below said designated release pressure, andsuch that pressure above said designated release pressure overcomes saidcapillary pressure and gas passes through said outer wrap.
 10. Theliquid filter according to claim 1 wherein said gas trap and pressureresponsive release mechanism delays rise in liquid level in said inletchamber for applications where said filter element is changedprematurely and has a longer life than otherwise indicated by saidrising liquid level in said inlet chamber.
 11. The liquid filteraccording to claim 1 wherein said inlet chamber is viewable through saidhousing such that an operator can see the level of liquid in said inletchamber.
 12. In a liquid filter comprising a housing having an inlet andan outlet, a filter element in said housing and having an upstream outerface communicating with said inlet, and a downstream inner facecommunicating with said outlet, said filter element comprising filtermedia performing a filtration function by passing liquid therethrough,said housing having a sidewall which defines with said outer face aninlet chamber therebetween having an upper section and a lower section,such that as liquid enters said inlet chamber from said inlet, gas insaid liquid rises to said upper section of said inlet chamber, such thatduring use, liquid level in said inlet chamber rises and said gasdisappears as permitted by gas flow through said filter media, therising of said liquid level and the disappearing of said gas being usedin the prior art for indicating a change interval for the filterelement, whereby said filter media provides both a filtration functionand a plugging indication function in the prior art, an improvedinterval change plugging indication method comprising separating saidfiltration function and said plugging indication function by trappinggas in said inlet chamber and releasing said gas in response to adesignated release pressure corresponding to a desired terminalpressure.
 13. The method according to claim 12 wherein during use, thepressure drop across said filter media increases as the latter becomesmore restrictive to liquid flow therethrough as more contaminant iscaptured, and wherein said designated release pressure is independent ofand does not vary with the increasing restriction of said filter mediato liquid flow therethrough.
 14. The method according to claim 12comprising performing said filtration function with a first memberprovided by said filter media, and performing said plugging indicationfunction with a second member different than said first member.
 15. Themethod according to claim 14 wherein the flow properties of said firstmember vary during filtration, and the flow properties of said secondmember do not substantially vary during filtration.
 16. The methodaccording to claim 14 comprising providing said second member by anouter wrap around said outer face around said filter element, said outerwrap having a lower permeability than said filter media, and calibratingsaid designated release pressure corresponding to said desired terminalpressure according to bubble point of said outer wrap.
 17. The methodaccording to claim 12 wherein said inlet chamber is viewable throughsaid housing, and said method is a gas visualization interval changeplugging indication method.
 18. A filter element for a liquid filterhaving a housing having an inlet and an outlet and housing said filterelement therein, said filter element having an upstream outer facecommunicating with said inlet, and a downstream inner face communicatingwith said outlet, said housing having a sidewall defining an inletchamber between said sidewall and said upstream outer face, whereinliquid is filtered by flowing from said inlet chamber through saidfilter element from said upstream outer face to said downstream innerface, said liquid giving off vapor gas within said housing, the higherthe level of said liquid in said inlet chamber the greater the pressuredrop across said filter element and the greater the plugging of saidfilter element, said filter element including a delay member fordelaying the rise in liquid level in said inlet chamber for applicationswhere said filter element is changed prematurely and has a longer lifethan otherwise indicated by said rising liquid level in said inletchamber.
 19. The filter element according to claim 18 wherein said delaymember comprises a change interval plugging indicator comprising a gastrap and pressure responsive release mechanism trapping said gas todelay the rise in liquid level in said inlet chamber for applicationswhere said filter element is changed prematurely and has a longer lifethan otherwise indicated by said rising liquid level, and releasing saidgas in response to a designated release pressure corresponding to adesired terminal pressure, to prevent premature plugging indicationotherwise indicated by said rising liquid level.
 20. The filter elementaccording to claim 19 wherein said gas trap and pressure responsiverelease mechanism traps said gas in said inlet chamber until saiddesignated release pressure corresponding to said desired terminalpressure, and then releases said gas to escape to said outlet, whichescaping gas is replaced by increasing liquid levels in said inletchamber, indicating that said designated release pressure correspondingto said desired terminal pressure has been reached, in turn indicatingthat replacement of said filter element is due.
 21. The filter elementaccording to claim 19 wherein said filter element comprises filter mediaperforming a filtration function filtering said liquid, and whereinduring use, the pressure drop across said filter media increases as thelatter becomes more restrictive to liquid flow therethrough as morecontaminant is captured, and wherein said designated release pressure isindependent of and does not vary with said increasing restriction ofsaid filter media to liquid flow therethrough.
 22. The filter elementaccording to claim 19 wherein said filter element comprises a firstmember provided by filter media performing a filtration functionfiltering said liquid, and a second member provided by said gas trap andpressure responsive release mechanism performing a plugging indicationfunction, said second member being different than said first member andseparating said plugging indication function from said filtrationfunction.
 23. The filter element according to claim 22 wherein the flowproperties of said first member vary during filtration, and the flowproperties of said second member do not substantially vary duringfiltration.
 24. The filter element according to claim 22 wherein saidsecond member comprises an outer wrap around said outer face and havinga lower permeability than said filter media, and wherein said designatedrelease pressure corresponding to said desired terminal pressure iscalibrated according to bubble point of said outer wrap.
 25. The filterelement according to claim 19 wherein said change interval pluggingindicator comprises an outer wrap around said outer face of said filterelement in said upper section of said inlet chamber and blocking gasflow therethrough at least at pressures below said designated releasepressure.
 26. The filter element according to claim 25 wherein saidouter wrap has a lower end which defines a lower end of said uppersection of said inlet chamber and an upper end of said lower section ofsaid inlet chamber, and wherein said gas is trapped in said uppersection of said inlet chamber when liquid in said inlet chamber risesabove said lower end of said outer wrap.
 27. The filter elementaccording to claim 26 wherein said outer wrap is a porous member wettedby said liquid such that capillary pressure in said porous member blocksgas flow therethrough below said designated release pressure, and suchthat pressure above said designated release pressure overcomes saidcapillary pressure and gas passes through said outer wrap.
 28. Thefilter element according to claim 26 wherein said filter element is anaxially extending annulus having an outer surface providing said outerface, and having an inner surface providing said inner face and defininga hollow interior, said filter element having a lower axial end havingan opening at said hollow interior and communicating with said outlet,said filter element having an upper axial end, and comprising a checkvalve at said upper axial end of said filter element, said check valvehaving a first side communicating with said hollow interior and having asecond side communicating with said upper section of said inlet chamber,said check valve having a closed condition below said designated releasepressure, and an open condition above said designated release pressure.29. The filter element according to claim 28 wherein said check valvecomprises porous media at said upper axial end covering said hollowinterior and blocking gas flow therethrough below said designatedrelease pressure, and passing gas flow there-through above saiddesignated release pressure.
 30. The filter element according claim 28wherein said check valve comprises a biased valve member having anormally closed condition below said designated release pressure, andhaving an open condition overcoming said bias above said designatedrelease pressure.
 31. The filter element according to claim 28 whereinsaid outer wrap is substantially nonporous.
 32. The filter elementaccording to claim 31 wherein said outer wrap has a permeabilitysubstantially lower than said filter element.
 33. The filter elementaccording to claim 18 wherein said inlet chamber is viewable throughsaid housing such that an operator can see the level of liquid in saidinlet chamber as an indication of when to replace said filter element.